The ultimate Sun weather Pokemon, especially with Chlorophyll.
+STAB Hits 7 types Super Effectively. +A Fire-Type with a real edge against Water. -STAB does nothing against other Fire types.
+One of only two Fire-Types not weak to Water. (Other is Reshiram) +Fire rids of Grass' Fire, Ice, and Bug weaknesses. +Grass rids of Fire's Water and Ground weakness. +Resists Grass, Electric, and Steel. +Gives Sun teams -Depending on how you look at it, as a Grass-Type, loses key resistances to Water and Ground. -Takes Neutral from a lot of things. -Weak to Poison, Flying, and Rock.
I was originally thinking that a Grass/Fire pokemon could be a Jack O'lantern styled pokemon. But that could be a Grass/Ghost pokemon.
The only other idea I had for a Grass/Fire type. Would be a Bushfire pokemon, but I realized that it would probably look like a Tangela on fire. Soooo yeah. I've no idea what they could use to make a Fire/Grass pokemon. But it would be really cool.
I'd also like to see a Electric/Fire type, Fighting/Flying, Psychic/Electric, Dark/Electric, and Electric/Fighting pokemon.
Sic vis pacem, para bellum. Don't make me HM01 you.